In order to determine if a mobile terminal should be handed over from a serving cell to a target cell, the mobile terminal may obtain a number of measurement samples of the signal power and/or the signal quality for signals exchanged with the serving cell and one or more candidate cells including the target cell. In general, when a target cell is perceived to provide signals of a higher power and/or quality than the serving cell, the mobile terminal may transmit a measurement report based upon the measurement samples from the serving cell and the candidate cells to the network to facilitate a determination as to whether the mobile terminal should be handed over to the target cell.
Because of the varying nature of radio signals and the movement of a mobile terminal, the received signal strength or quality may fluctuate. To avoid handover failure, e.g., radio link failure, or handover oscillation in which a mobile terminal is repeatedly handed back and forth between a pair of cells, the determination as to whether a handover is merited may be based, not on a temporary fluctuation in radio signal power or quality, but on a longer term average or trend of the radio signal power or quality. As such, a mobile terminal may obtain a series of measurements of the received signal strength or quality from various cells over the course of time. In order to reduce the possibility of handover failure or handover oscillation, several handover parameters may be defined to govern the repeated measurement of the received signal strength or quality. These handover parameters include time to trigger (TTT) and handover margin. The TTT is defined as the minimum time that the condition that is required to trigger the handover, such as the received signal power or quality of a target cell exceeding that of the serving cell, must be maintained for a measurement report to be generated. By requiring the conditions be maintained for the TTT, the mobile terminal may cause initiation of a handover in an instance in which the triggering condition is sustained, but not in an instance in which the triggering condition is only temporarily or transiently set aside. In order to continue to monitor the received signal strength or quality throughout the TTT period, the mobile terminal must generally obtain a number of measurement samples throughout the TTT period.
In order to conserve power, discontinuous reception (DRX) techniques have been developed. A mobile terminal generally implements a discontinuous reception technique by having periods in which the mobile terminal monitors network communications, such as by monitoring the physical downlink control channel (PDCCH). During these periods, the mobile terminal may obtain measurement samples, such as of the received signal strength or quality of signals from the serving cell and various candidate cells. The periods in which the mobile terminal monitors the network communications are separated by a sleep period during which the mobile terminal does not monitor the network communications and, instead, conserves power.
As shown, for example, in FIG. 1, the sleep period may be defined by a DRX timer. Upon expiration of the DRX timer, e.g., DRX timeout, the mobile terminal may be configured to monitor network communications, such as by monitoring the PDCCH. The mobile terminal may be configured to monitor network communications for a predetermined period of time identified in FIG. 1 as the monitoring period. During the monitoring period, the mobile terminal may be scheduled with new data, such as the transmission or reception of data via the uplink or the downlink, respectively. In this instance, the monitoring period during which the mobile terminal will monitor network communications shall be extended by a predetermined time period as measured by an inactivity time. The monitoring period may be extended several times in an instance in which the mobile terminal is repeatedly scheduled with new data. As shown in FIG. 1, for example, the monitoring period is extended twice as a result of the mobile terminal twice being scheduled with new data. The first extension of the monitoring period is shown with dashed lines, while the second extension of the monitoring period is shown with dotted lines. Once either the initial monitoring period or an extension of the monitoring period ends without the mobile terminal being scheduled with new data, the mobile terminal may enter a sleep mode for a period of time governed by the DRX timer. As shown in FIG. 1, the sleep period is measured from the end of the initial monitoring period such that any extension of the monitoring period effectively reduces the sleep period. By alternately monitoring the network communications and sleeping, the power consumption of the mobile terminal may be reduced.
In an instance in which a measurement sample is obtained during the period in which the mobile terminal is monitoring network communications and the measurement sample is such that that the TTT period is commenced, such as a measurement sample in which the received signal power or quality of a signal from a candidate cell is greater than that of a signal from the serving cell, the mobile terminal may obtain additional measurement samples to determine if a handover is advisable. The monitoring period may be sufficiently long to obtain the additional measurement samples that are desired during the TTT period to facilitate the determination regarding handover. However, in some instances, the monitoring period may conclude without the mobile terminal having collected any additional measurement samples or without collecting a sufficient number of additional measurement samples so as to determine if a handover is advisable.
In this instance, the mobile terminal may enter into a sleep period and a determination regarding handover either must be made based upon a fewer number of measurement samples than is desired which, in turn, may lead to handover in instances in which a handover is not desirable, such as in instances in which the initial measurement sample turns out to be aberrational or otherwise not representative of the received signal power or quality of the serving and target cells. As a result of the entry into the sleep period, however, the mobile terminal may be unable to obtain additional measurement samples in order to more appropriately determine if a handover is appropriate. Alternatively, the mobile terminal may wait until the next monitoring period to obtain additional measurement samples which may disadvantageously delay any handover.